We have demonstrated that iodine exchange reactions occur in in vitro systems used for studying iodination reactions. Experiments are planned to test whether such exchange reactions occur in vivo in the thyroid gland; if they do, it is important to be aware of this in evaluating experiments in which hormonogenesis in the thyroid is judged from experiments using radioactive iodine. Experiments will also be done to isolate a CAMP-dependent kinase enzyme from rat thyroid tissue, to study its properties, and to see if the activity of this enzyme can be correlated with various physiological states of the animal.